Post-treatment method of film-coated member

ABSTRACT

Provided is a post-treatment method of a film-coated member, the method including heating an electrically conductive film provided on at least one surface of a substrate by applying pressure to a member including the substrate and the film and applying a high-frequency direct current pulse to the film, wherein the pressure is applied in a direction having a normal-vector-direction component of a layer of the film.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims the benefit of Korean Patent Application No.10-2015-0014407, filed on Jan. 29, 2015, in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein in itsentirety by reference.

BACKGROUND

1. Field

The present invention relates to a post-treatment method of afilm-coated member and, more particularly, to a post-treatment method ofa film-coated member as a component facing a nuclear fusion reactor.

2. Description of the Related Art

About 80% or more of world energy consumption currently depends onfossil fuels and no significant change thereof is expected for the nextfew decades. According to a report of BP (British Petroleum), there areonly about 1.3331 trillion barrels of oil (45.7 years), about 187.5trillion m³ of natural gas (62.8 years), and about 826 billion tons ofcoal (119 years) in recoverable deposits based on 2009. Besides, uraniumwhich is the most commonly used to generate nuclear power will beexhausted after about 70 years. In spite of the resources close toexhaustion, global energy demand will continuously increase.Particularly, a rapid increase in energy demand accompanied witheconomic growth is expected in large countries such as China, India, andBrazil.

Meanwhile, Korea highly depends on fossil fuels to generate electricityand imports crude oil mostly from the Middle East. Since crude oil hasinstable supply and demand and high price fluctuations depending on theinternational situation, a solution for stably securing energy is deeplyneeded. Furthermore, fossil fuels are regarded as the cause of anenvironmental disaster in terms of carbon dioxide emissions. Developmentof new clean energy sources for replacing the fossil fuels is highlydemanded, and new renewable energy such as solar heat/light energy,biomass energy, and wind energy are be actively developed.

However, the new renewable energy has a low energy conversionefficiency, is highly instable depending on natural conditions, and thusis not suitable for the demand. Nuclear fusion energy is regarded as oneof solutions thereof. The nuclear fusion energy uses seawater, which ishardly exhausted, as a fuel and is safe compared to nuclear fission.However, a reactant for nuclear fusion should be made to plasma state,and heating is needed to an ultra-high temperature above about 100million ° C. to generate high-performance plasma and to effectivelycalculate the amount of output energy.

In addition, a technology related to an extreme material capable ofwithstanding an ultra-high temperature and a high vacuum state is neededfirst. Tungsten attracts people's attention as a plasma-facing materialin a tokamak which is a device for sealing plasma therein. However,tungsten is brittle at a low temperature, and has a high density interms of structural load and thus should be used together with anotherstructural material.

PRIOR ART Patent Document

(Patent document 1) Korean Patent No. 10-1459051 (Oct. 31, 2014)

SUMMARY

The present invention provides a post-treatment method of a film-coatedmember to improve mechanical properties thereof. However, the scope ofthe present invention is not limited thereto.

According to an aspect of the present invention, there is provided apost-treatment method of a film-coated member. The method includesapplying a pressure to a member including a substrate and anelectrically conductive film on at least a surface of the substrate; andheating the film by applying a high-frequency direct current pulse tothe film, wherein the pressure is applied in a direction having acomponent along a direction of a normal vector to a layer on which thefilm is disposed.

The film may be formed by thermal spraying.

The thermal spraying scheme may include at least one of gas flamethermal spraying, arc thermal spraying, plasma thermal spraying,detonation thermal spraying, vacuum plasma spraying, and high velocityoxy-fuel spraying.

The film may include tungsten.

The heating may include heating the film to a temperature of about 900°C. to about 1100° C.

The heating may include applying a pulsed current of up to 4000 A and 20kHz.

The substrate may be an electrical conductor, and electrodes forapplying the high-frequency direct current pulse may be provided in sucha manner that one of the electrodes contacts the substrate and the otherof the electrodes contacts the film.

The heating may include applying a pulsed current having a currentdensity from 50 A/mm² to 250 A/mm², to the film.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail embodiments thereofwith reference to the attached drawings in which:

FIG. 1 is a schematic diagram of a post-treatment apparatus used in apost-treatment method of a film-coated member, according to anembodiment of the present invention;

FIG. 2 is a flowchart of a post-treatment method of a film-coatedmember, according to an embodiment of the present invention; and

FIGS. 3, 4 and 5 are images showing results of analyzing samplesimplemented using a post-treatment method of a film-coated member,according to test examples and comparative examples of the presentinvention.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments, examples of whichare illustrated in the accompanying drawings. However, embodiments arenot limited to the embodiments illustrated hereinafter, and theembodiments herein are rather introduced to provide easy and completeunderstanding of the scope and spirit of embodiments. In the drawings,the thicknesses of layers and regions are exaggerated for clarity.

It will be understood that when an element, such as a layer, a region,or a substrate, is referred to as being “on,” “connected to” or “coupledto” another element, it may be directly on, connected or coupled to theother element or intervening elements may be present. In contrast, whenan element is referred to as being “directly on,” “directly connectedto” or “directly coupled to” another element or layer, there are nointervening elements or layers present. Like reference numerals refer tolike elements throughout. As used herein, the term “and/or” includes anyand all combinations of one or more of the associated listed items.

It will be understood that, although the terms first, second, third,etc., may be used herein to describe various elements, components,regions, layers and/or sections, these elements, components, regions,layers and/or sections should not be limited by these terms. These termsare only used to distinguish one element, component, region, layer orsection from another element, component, region, layer or section. Thus,a first element, component, region, layer or section discussed belowcould be termed a second element, component, region, layer or sectionwithout departing from the teachings of embodiments.

Spatially relative terms, such as “above,” “upper,” “beneath,” “below,”“lower,” and the like, may be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the term “above” may encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of embodiments. Asused herein, the singular forms “a,” “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Embodiments of the invention are described herein with reference toschematic illustrations of idealized embodiments (and intermediatestructures) of the invention. As such, variations from the shapes of theillustrations as a result, for example, of manufacturing techniquesand/or tolerances, are to be expected. Thus, the embodiments of theinvention should not be construed as limited to the particular shapes ofregions illustrated herein, but are to include deviations in shapes thatresult, for example, from manufacturing.

FIG. 1 is a schematic diagram of a post-treatment apparatus 1000 used ina post-treatment method of a film-coated member, according to anembodiment of the present invention.

Referring to FIG. 1, in the post-treatment apparatus 1000 according toan embodiment of the present invention, a thermal-sprayed circularspecimen 100 may be put in a mold 110 and then two electrodes 210 mayindividually contact top and bottom surfaces of the specimen 100. Themold 110 is formed of, for example, a graphite carbon material, and hasa cylindrical hole to put the specimen 100 therein. The specimen 100 maybe produced by providing a film 20 on a substrate 10. The electrodes 210may be formed of, for example, the same material as the mold 110, i.e.,the graphite material, and may have a size to be put in the hole of themold 110. The mold 110 and the electrodes 210 are cylindrical in FIG. 1but may have another shape depending on the shape of the specimen 100.

In addition, after the specimen 100 is put in the mold 110, one of theelectrodes 210 may contact the substrate 10, the other of the electrodes210 may contact the film 20, electrode pressers 200 of a spark plasmasintering device 900 may apply pressure thereto based on each condition,and a power supplier 300 may apply a high-frequency direct current pulseto the specimen 100 to heat the film 20 of the specimen 100. Forexample, a pulsed current having a current density from 50 A/mm² to 250A/mm² may be applied to the film 20 to heat the film 20.

FIG. 2 is a flowchart of a post-treatment method of a film-coatedmember, according to an embodiment of the present invention.

Referring to FIG. 2, the post-treatment method according to anembodiment of the present invention includes forming a film on asubstrate using plasma thermal spraying (S100), and heating the film byapplying pressure and a high-frequency direct current pulse to a memberincluding the film (S200).

A detailed description is now given of the post-treatment method. Amember including an electrically conductive film may be produced using athermal spraying scheme on an electrical conductor substrate. Thethermal spraying scheme may use at least one of, for example, gas flamethermal spraying, arc thermal spraying, plasma thermal spraying,detonation thermal spraying, vacuum plasma spraying, and high velocityoxy-fuel spraying.

The film may be heated by applying the high-frequency direct currentpulse to the film while applying pressure to the member including theelectrically conductive film produced using the above method. In thiscase, the pressure may be applied in a direction having anormal-vector-direction component of a layer of the film. Herein, thenormal-vector-direction component refers to a non-zero component.Assuming that the film is in a horizontal layer, the direction ofapplying the pressure may be a direction perpendicular to the layer ofthe film.

That is, for example, a specimen may be produced byplasma-thermal-spraying tungsten powder on a graphite substrate usinghelium and hydrogen as a plasma induced gas. The substrate may use agraphite substrate. The specimen may be put in a mold to contactelectrodes, mounted in a spark plasma sintering device, and then pressedwith a pressure of about 2 kN to 3 kN.

In this case, one of the electrodes for applying a high-frequency directcurrent pulse contacts the graphite substrate and the other contacts thetungsten film. As such, a pulsed current of up to 4000 A, 20 kHz may beapplied to the tungsten film and thus the tungsten film may be heated toa temperature of about 900° C. to 1100° C. After reaching a settemperature, the tungsten film may be maintained at the set temperaturefor a certain period of time and then cooled.

A description is now given of test examples to which the above-describedtechnical idea is applied. However, the following test examples aregiven only for a better understanding of the present invention and thepresent invention is not limited thereto.

FIGS. 3 to 5 are images showing results of analyzing surfacemicrostructures and hardness of samples implemented using apost-treatment method of a film-coated member, according to testexamples and comparative examples of the present invention.

Referring to FIGS. 3 to 5, samples 1 to 8 correspond to the comparativeexamples of the present invention, and samples 9 to 14 correspond to thetest examples of the present invention. Specimens produced byplasma-thermal-spraying a tungsten film to a thickness of about 200 μmon a graphite substrate are used in all cases.

Specifically, sample 1 is a reference specimen produced byplasma-thermal-spraying tungsten powder on a graphite substrate withoutperforming post-treatment, and optical microscope, scanning electronmicroscope, and hardness tests are performed thereon to check basicinformation of the reference specimen. Sample 2 is a specimen producedby performing vacuum heat treatment while maintaining a vacuum level of10⁻⁴torr at about 900° C. for about 1 hour. Samples 3 to 8 are specimensproduced by applying pressures of 2 kN and 3 kN in a depositiondirection at about 900° C., 1000° C., and 1100° C. and maintaining thetemperatures for about 10 minutes before cooling. Pressure and heattreatment is performed on the specimens by setting a heating rate toabout 100° C./min at about 1 atmosphere of an argon (Ar) gas.

Meanwhile, samples 9 to 14 are specimens produced by applying ahigh-frequency direct current pulse to the samples to generateresistance heat in the specimens to perform heat treatment. A sparkplasma sintering device is used to form an environment for allowing thepulsed current to flow through the specimens, and pressures of about 2kN and 3 kN are individually applied to allow electrodes to contact thespecimens to flow the current therethrough. In this case, finaltemperatures of the specimens are about 900° C., 1000° C., and 1100° C.,and are maintained for about 10 minutes before cooling.

FIG. 3 shows results of analyzing microstructures of surfaces of thesamples using an optical microscope, FIG. 4 shows results of analyzingmicrostructures of surfaces of the samples using a scanning electronmicroscope. Referring to FIGS. 3 and 4, based on characteristics ofthermal spray coating, splats of molten powder are sequentiallyaccumulated on the substrate due to a spray gas and thus cracks areshown between the splats, which are called splat boundaries.

Since the splat boundaries inhibit overall properties of the tungstenfilm, heat treatment is performed to eliminate the same. The splatboundaries may be observed using an optical microscope and may also beclearly viewed using a scanning electron microscope. When tungstensplats are accumulated on the substrate, due to a large difference intemperature between the substrate and the molten tungsten splats, thespeed at which a melting point is transferred to the inside is less thanthe speed at which crystal grains grow. Accordingly, a crystal growthdirection is determined and the crystal structure inside one splat formsa columnar structure. Since solidification occurs instantaneously andrapidly due to contact with the substrate, a sufficient time to growcrystals having a preferred orientation may not be secured.

Furthermore, a microstructure of sample 1 is compared to those of theother samples in terms of splat boundaries. When a rate of splatboundaries corresponding to empty spaces between splats is qualitativelycompared, sample 2 produced by performing vacuum heat treatment at about900° C. for 1 hour has a microstructure similar to that of sample 1.Samples 3 to 8 produced by performing pressure and heat treatment asanother post-treatment method, and samples 9 to 14 produced based onpost treatment performed by applying a direct current pulse show thatregions observed as splat boundaries on the optical images areconsiderably reduced.

Meanwhile, referring to samples 3 to 8 and samples 9 to 14 shown in FIG.3, when pressure and heat treatment is equally performed as posttreatment, regions of splat boundaries are reduced and gaps betweensplats are also reduced if the temperature is higher or if the appliedpressure is higher under the same temperature condition. In addition,when the microstructures are compared between the simple pressure andheat treatment method and the post-treatment method for applying adirect current pulse to generate heat under the same temperature andpressure condition, splat boundaries are eliminated a lot from themicrostructure produced using the post-treatment method for applying adirect current pulse.

In addition to the above optical microscope image analysis results, thescanning electron microscope image analysis results of FIG. 4 also showthat a heat treatment method for directly applying a high-frequencydirect current pulse to a specimen is effective to reduce spaces ofsplat boundaries.

Meanwhile, compared to sample 1, most post-treated specimens show thatno preferred orientation is generated or no change in crystaldistribution occurs in the tungsten film, because a temperature range ofabout 900° C. to 1100° C. is not sufficiently high to causere-crystallization or recovery of tungsten. In general, it is known thata re-crystallization temperature of metal is located between ⅓ and ½ ofa melting point thereof and that a re-crystallization temperature oftungsten is located in a temperature range of about 1300° C. to 1500° C.As such, since the temperature condition range of the current testexamples and the comparative examples does not reach are-crystallization temperature, a noticeable change in crystaldistribution does not occur in electron backscattered diffractionanalysis.

Furthermore, it is reported that a spark plasma sintering scheme is moreeffective to sinter a powdered material compared to a general sinteringscheme or a hot pressing scheme because localized necking occurs on finecontact surfaces between powder grains due to Joule heating and thusdensification of the material is caused within a short time.

In the current test examples, since the tungsten film of the specimen isprovided in the form of accumulation of splats, the splats may beregarded as powder grain entities. In this case, pressure and DCelectricity are applied to provide a high-frequency pulsed current,temperature is locally concentrated on small contact areas betweensplats due to Joule heating to reduce the area of splat boundaries, andthus the microstructure of the film becomes more dense.

FIG. 5 is a graph showing results of measuring hardness of the samples.Referring to FIG. 5, due to continuous reception of plasma particles andheat, a plasma-facing component has a high probability of corrosion andthus the life thereof is shortened. Various mechanical properties suchas toughness, ductility, and hardness may be used to evaluate resistanceagainst exposure to plasma particles for a long life.

However, the current tests are performed to aim a reference hardnessvalue recommended for an international thermonuclear experimentalreactor (ITER) and a demonstration power plant (DEMO). The recommendedhardness value is HV 30 and a load of about 300N should be applied.However, since the height of a tungsten film coated using a currentlyoptimized plasma thermal spraying scheme to have durability is about 200um, if the load of about 300N is applied, the load far exceeds thisthickness and thus is not appropriate to evaluate properties of only thetungsten film. As such, the current tests apply a load of up to about0.5N in consideration of the size of indentations to compare allsamples.

The measured hardness is 122 HV (a standard deviation is 27). Althoughthe hardness of a tungsten block other than the thermal-sprayed tungstenfilm is variable depending on a scheme of producing the block or thetype of tungsten powder used to produce the block, the hardness of ablock produced using raw material powder of the tungsten film of thereference specimen is about 502 HV, and the hardness of the tungstenfilm of the reference specimen of a plasma-facing component may be about¼ compared to the bulk block.

The reason why the hardness of the plasma thermal-sprayed tungsten filmis lower than that of bulk tungsten may include the influence of splatboundaries. Dispersion of splat boundaries on a microstructure exerts abad influence on overall mechanical properties of the film andconsiderable amounts of bubbles and cracks are observed to reducedensity.

The hardness value of sample 1 is compared to those measured aftervarious post-treatment processes. Micro indentation tests are performedon each specimen at an interval equal to or greater than about 100 umafter polishing a cross section of the specimen, and results thereof areas described below. Average hardness values of the tungsten films ofsamples 1 and 2 are 134.66 HV and 134.97 HV, and standard deviationsthereof are 13.68 HV and 9.95 HV, respectively.

The hardness values of the tungsten films of sample 1 produced withoutperforming post-treatment, sample 2 produced by performing vacuum heattreatment at about 900° C. for 1 hour, and samples 3 to 8 produced byapplying pressures of 2 kN and 3 kN in a deposition direction of thespecimens at about 900° C., 1000° C., and 1100° C. are shown in FIG.5(a).

In consideration of the size of the samples, the forces of 2 kN and 3 kNcorrespond to pressures of 25.47 MPa and 38.21 MPa, respectively. It isshown that the hardness increases if the heat treatment temperature ishigh and if the applied pressure is high. When the heat treatmenttemperature is 900° C., the hardness values corresponding to the appliedpressures of 25.47 MPa and 38.21 MPa are 167.83 HV and 167.87 HV, andstandard deviations thereof are 30.74 HV and 22.02 HV, respectively.When the heat treatment temperature is 1000° C., the hardness valuescorresponding to the applied pressures are 259.26 HV and 276.15 HV, andstandard deviations thereof are 46.25 HV and 54.32 HV, respectively.When the heat treatment temperature is 1100° C., the hardness valuescorresponding to the applied pressures are 282.02 HV and 286.95 HV, andstandard deviations thereof are 59.32 HV and 66.22 HV, respectively.Although the ranges of the standard deviations overlap between theprocessing conditions, it is shown that an increase in temperature orpressure is related to an increase in hardness based on the tendency ofaverage values.

In addition, the hardness values of the tungsten films of sample 1produced without performing post-treatment, sample 2 produced byperforming vacuum heat treatment at about 900° C. for 1 hour, andsamples 9 to 14 produced by applying pressures of 2 kN and 3 kN in adeposition direction of the specimens and applying a high-frequencydirect current pulse at about 900° C., 1000° C., and 1100° C. are shownin FIG. 5(b).

Similarly to the case of pressure and heat treatment, the hardnessincreases if the heat treatment temperature is high and if the appliedpressure is high. When the heat treatment temperature is about 900° C.,the hardness values measured at the pressures of 25.47 MPa and 38.21 MPaare 212.12 HV and 247.35 HV in average, and standard deviations thereofare 56.75 HV and 32.02 HV, respectively. When the heat treatmenttemperature is 1000° C., the average hardness values are 340.25 HV and358.52 HV, and standard deviations thereof are 41.80 HV and 56.52 HV,respectively. When the heat treatment temperature is 1100° C., theaverage hardness values are 363.16 HV and 363.11 HV, and standarddeviations thereof are 46.90 HV and 23.87 HV, respectively.

When the post-treatment method for performing pressure and heattreatment is compared to the post-treatment method for applying ahigh-frequency direct current pulse using a spark plasma sinteringdevice, if post-treatment is performed at the same temperature byapplying the same pressure for the same period of time, the hardness ofthe tungsten film is increased more in the method for applying ahigh-frequency direct current pulse compared to the post-treatmentmethod for performing pressure and heat treatment using externalheating. In this case, an average hardness value of bulk tungsten ismeasured to about 501 HV when a load of up to about 0.5N is applied asin the above tungsten film test. More than 70% of the hardness of bulktungsten may be achieved as the hardness of the tungsten film using thepost-treatment method for directly applying a direct current pulse to aspecimen.

By slowing down corrosion of a component facing a nuclear fusion reactorto be exposed to various plasma particles, a cycle of replacing atungsten film may be increased and the amount of tungsten by-productsgenerated due to corrosion and serving as impurities in reaction plasmamay be reduced.

A film member implemented using the above-described post-treatmentmethod of a film-coated member, according to an embodiment of thepresent invention, is merely an example of a component facing a nuclearfusion reactor, and the technical idea of the present invention is notlimited thereto. Accordingly, due to an excellent corrosion resistanceand a high hardness value, the film member is applicable as a filmmember used in a highly corrosive environment as well as the componentfacing a nuclear fusion reactor.

As described above, a post-treatment method of a film-coated member,according to an embodiment of the present invention, may densify amicrostructure of the film and may improve mechanical properties of acomponent facing a nuclear fusion reactor. However, the scope of thepresent invention is not limited to the above effect.

While the present invention has been particularly shown and describedwith reference to embodiments thereof, it will be understood by those ofordinary skill in the art that various changes in form and details maybe made therein without departing from the spirit and scope of thepresent invention as defined by the following claims.

What is claimed is:
 1. A post-treatment method of a film-coated member,the method comprising: applying a pressure to a member including asubstrate and an electrically conductive film on at least a surface ofthe substrate; and heating the film by applying a high-frequency directcurrent pulse to the film, wherein the pressure is applied in adirection having a component along a direction of a normal vector to alayer on which the film is disposed.
 2. The post-treatment method ofclaim 1, wherein the film is formed by thermal spraying.
 3. Thepost-treatment method of claim 2, wherein the thermal spraying includesat least one of gas flame thermal spraying, arc thermal spraying, plasmathermal spraying, detonation thermal spraying, vacuum plasma spraying,and high velocity oxy-fuel spraying.
 4. The post-treatment method ofclaim 1, wherein the film includes tungsten.
 5. The post-treatmentmethod of claim 1, wherein the heating comprises heating the film to atemperature of about 900° C. to about 1100° C.
 6. The post-treatmentmethod of claim 1, wherein the heating comprises applying a pulsedcurrent of up to 4000 A and 20 kHz.
 7. The post-treatment method ofclaim 1, wherein the substrate is an electrical conductor, and whereinelectrodes for applying the high-frequency direct current pulse areprovided in such a manner that one of the electrodes contacts thesubstrate and the other of the electrodes contacts the film.
 8. Thepost-treatment method of claim 1, wherein the heating comprises applyinga pulsed current having a current density from 50 A/mm² to 250 A/mm², tothe film.